1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to a system for imaging lithographic printing plates on- or off-press using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, an image to be transferred to a recording medium is represented on a plate, mat or other printing member as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the member is simply inked and the image transferred onto a recording material; the member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic in the sense of affinity for dampening (or "fountain") solution, and the necessary ink-repellency is provided by an initial application of such a solution to the plate prior to inking. The ink-abhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
If a press is to print in more than one color, a separate printing plate corresponding to each color is required. The plates are each mounted to a separate plate cylinder of the press, and the positions of the cylinders coordinated so that the color components printed by the different cylinders will be in register on the printed copies. Each set of cylinders associated with a particular color on a press is usually referred to as a printing station.
Because of the ready availability of laser equipment and their amenability to digital control, significant effort has been devoted to the development of laser-based imaging systems. Early examples utilized lasers to etch away material from a plate blank to form an intaglio or letterpress pattern. See, e.g., U.S. Pat. Nos. 3,506,779 and 4,347,785. This approach was later extended to production of lithographic plates, for example, by removal of a hydrophilic surface to reveal an oleophilic underlayer. See, e.g., U.S. Pat. No. 4,054,094. These systems generally require high-power lasers, which are expensive and slow.
A second approach to laser imaging involves the use of thermal-transfer materials. See, e.g., U.S. Pat. Nos. 3,945,318; 3,962,513; 3,964,389; 4,395,946, 5,156,938; and 5,171,650, as well as copending application Ser. No. 08/376,766. With these systems, a polymer sheet transparent to the radiation emitted by the laser is coated with a transferable material. During operation the transfer side of i this construction is brought into contact with an acceptor sheet, and the transfer material is selectively irradiated through the transparent layer. Irradiation causes the transfer material to adhere preferentially to the acceptor sheet. The transfer and acceptor materials exhibit different affinities for fountain solution and/or ink, so that removal of the transparent layer together with unirradiated transfer material leaves a suitably imaged, finished plate. Typically, the transfer material is oleophilic and the acceptor material hydrophilic. This technique generally requires maintenance of a highly clean environment to avoid image degradation.
Lasers can also be used to expose a photosensitive blank for traditional chemical processing. See, e.g., U.S. Pat. Nos. 3,506,779; 4,020,762. Similalry, lasers have been employed to selectively remove, in an imagewise pattern, an opaque coating that overlies a photosensitive plate blank. The plate is then exposed to a source of radiation, with the unremoved material acting as a mask that prevents radiation from reaching underlying portions of the plate. See, e.g., U.S. Pat. No. 4,132,168. Either of these imaging techniques requires the cumbersome chemical processing associated with traditional, non-digital platemaking.
More recently, lithographic printing plates have been designed for low-power ablation imaging mechanisms. U.S. Pat. Nos. 5,339,737 and 5,379,698 (the entire disclosures of which are hereby incorporated by reference) disclose a variety of ablation-type lithographic plate configurations for use with imaging apparatus that utilize diode lasers. For example, laser-imageable lithographic printing constructions in accordance with these patents may include a first, topmost layer chosen for its affinity for (or repulsion of) ink or an ink-abhesive fluid; an ablation layer, which volatilizes into gaseous and particulate debris in response to imaging (e.g., infrared, or "IR") radiation, thereunder; and beneath the imaging layer, a strong, durable substrate characterized by an affinity for (or repulsion of) ink or an ink-abhesive fluid opposite to that of the first layer. Ablation of the imaging layer weakens the topmost layer as well. By disrupting its anchorage to an underlying layer, the topmost layer is rendered easily removable in a post-imaging cleaning step, creating an image spot having an affinity for ink or an ink-abhesive fluid differing from that of the unexposed first layer.
Although this type of construction facilitates much faster imaging and at power levels significantly lower than those of older "etching" laser systems, the laser pulse must still transfer sufficient energy to cause the ablation layer to catastrophically overheat and change phase. Accordingly, even low-power lasers must be capable of very rapid rise times, and imaging speeds--that is, the laser pulse rate--must not be so fast as to preclude the requisite energy buildup during each imaging pulse.
Microscopic observation of behavior during imaging of these three-layer constructions reveals that the initial response to a laser pulse is formation of a gas pocket between the surface layer and the underlying layer, which persists well after the pulse has terminated. This pocket is believed to be formed primarily by gas resulting from thermal decomposition of the surface layer immediately in contact with the underlying layer.
For example, investigations of dry plates in accordance with the '698 patent (comprising a polyester substrate, a titanium layer approximately 30 nm thick, and a silicone surface layer) suggest that the silicone layer debonds from the underlying titanium layer at laser fluences far short of that necessary for ablation of the titanium. This observation is important to understanding of the ablation mechanism. The polymeric layers above and below the titanium layer have substantially greater heat capacities than the very thin titanium, with the result that they act as heat sinks, dissipating laser energy absorbed by the titanium layer and thereby increasing the fluence necessary for ablation. With the titanium layer detached from the overlying silicone layer, however, heat dissipation is essentially halved, forcing the titanium layer to retain more of the laser energy. This observation validates the general preference for short, intense laser pulses, since these minimize heat transport (which is time-dependent) and also the fluence necessary to achieve ablation.
Unfortunately, this mechanism suggests the continued need for complete ablation of the titanium layer, with the consequent constraints on laser power and imaging speed. Unless the layer underlying the silicone is ablated, the silicone will reattach to that layer once the gas pocket has dissipated, and therefore will not be removed by mechanical cleaning processes.